1. Field of the Invention
This invention relates to thermocouples for sensing temperature, and in particular to thermocouples for sensing the presence and absence of a flame from a burner, and to generate EMF to operate safety devices.
2. Description of the Prior Art
The prior art as exemplified in U.S. Pat. Nos. 3,155,143 and 3,615,872 and in FIGS. 1, 2 and 3 includes a number of thermocouple constructions for sensing a flame from a burner.
In FIG. 1, a thermocouple construction indicated generally at 10 is fastened to a bracket indicated generally at 12 which also supports a burner such as a pilot burner indicated generally at 14. The thermocouple 10 has a pair of coaxial elements such as a center rod 16 and an outer tube 18 which are welded together at 20 to form a hot junction. The rod 16 and tube 18 are made from dissimilar metals such as constantan, an alloy containing essentially copper and nickel, and stainless steel, an alloy containing essentially steel and chromium, respectively. The other end of the center element 16 is joined to a conductor, such as being welded to the stripped end of a insulated copper wire 22 to form a junction 24. The tubular element 18 extends from the junction 20 over the element 16, the junction 24, and a portion of the conductor 22 and terminates in a flared end 26 which is joined such as by soldering to a conductor such as a tubular copper conductor 28 coaxial with the insulated wire conductor 22 to form a junction 30. A tubular insulator 32 is disposed between the elements 16 and 18 and extends over the junction 24 and the bare end of the wire conductor 22 to prevent electrical contact to the outer element 18. An annular groove 34 is engaged by a spring clip 36 to retain the thermocouple 10 at a selected position through respective holes in upper and lower flanges of the bracket 12. As illustrated the thermocouple 10 is positioned such that a flame from the burner 14 impinges on the outer tubular element 18 immediately adjacent the junction 20. Thus the junction 20 forms a hot junction of the thermocouple while the junctions 24 and 30 form cold junctions of the thermocouple.
In another prior art arrangement as illustrated in FIG. 2, several parts are identified by the same numbers as parts in FIG. 1 indicating that such commonly identified parts have substantially identical structure and/or function. A thermocouple indicated generally at 40 is mounted in a bracket indicated generally at 42 which supports the burner 14. The thermocouple 40 has an inner element 44 such as a rod of constantan, and an outer element, such as stainless steel tube 46 coaxial with the inner element 44 and joined together at one end to form the hot junction 20. The lower end of the inner element 44 is joined at a junction 48 to the bared end of the insulated wire 22. The lower end of the tubular element 46 is joined at 49, such as by soldering, to the upper end of a coupling member 50 which has a generally tubular configuration coaxial with the inner element 44 and is formed from brass. The coupling member 50 extends over the junction 48 to its lower end where it is joined, such as by soldering, at a junction 52 to the tubular conductor 28. The junctions 48, 49 and 52 form cold junctions for the thermocouple 40. An insulative sleeve 32 covers the junction 48 and the conductors on either side to prevent electrical shorting in the area of the junction 48. The member 50 has a plurality of annular radiating fins 56 formed therearound and spaced throughout substantial portions of the length of the member 50. An annular rib 57 is disposed around the member 50 for engaging the bottom of an upper flange 58 of the bracket 42 around a hole 60 in the flange 58 through which the member 50 extends. A second annular rib 62 is disposed around the member 50 for being engaged by a nut 64 threaded into a lower flange 66 on the bracket 42. The ribs 57 and 62, the opening 60, the flanges 58 and 66, and the nut 64 are such that the thermocouple 40 is accurately positioned in the bracket 42 relative to the burner 14.
Alternately, the thermocouple 40 is secured to the bracket 42 by other means such as the spring clip described in the aforementioned U.S. Pat. No. 3,615,872 instead of the nut 64.
The thermocouple construction 40 produces a substantially higher output EMF or voltage in the conductors 22 and 28 than the thermocouple construction 10 of FIG. 1. It is noted that the inner element 44 of the thermocouple 40 is substantially longer than the inner element 16 of the thermocouple 10 and that the outer element 46 of the thermocouple 40 is substantially shorter than the outer element 18 of the thermocouple 10. However, the cost of the thermocouple 40 is substantially higher than the cost of the thermocouple 10 due to the member 50 and the labor involved in making the junctions 49 and 52. The higher voltage output of the thermocouple 40 allows for less critical design of electromagnets, energy cut-off switches, meters, and other parts of safety valves or control devices which utilize the current from the thermocouples to maintain the flow of fuel during the presence of a flame from the burner 14 and to stop the flow of fuel during the absence of a flame from the burner 14.
In another thermocouple indicated generally at 70 in FIG. 3, an inner element 72 of constantan is welded at hot junction 20 to a stainless steel sleeve 74 which is coaxial with the inner element 72. An enlarged upper portion 76 of the outer copper conductor 28 is telescoped over and brazed to the lower portion of the sleeve 74 to form a junction 78. The inner conductor 22 is welded to the inner member 72 at the junction 80 within the enlarged portion 76. A tubular steel sheath 82 is attached to the portion 76 of outer conductor 28 by crimping the upper end of the sheath 82 into secure engagement with the portion 76 where it is telescoped over the sleeve 74. The sheath 82 extends completely over the enlarged portion 76 and an upper portion of the outer conductor 28 below the portion 76, and has an upper annular step 84 and a lower annular step 86 which are believed to be bracket engaging and nut engaging steps respectively. It is believed that the sheath 82 is nothing more than an adapter designed for attaching the thermocouple 70 to a selected size and shape of bracket since the sheath 82 has a smooth exterior with no apparent attempt to provide improved heat dissipation.